Field of the Invention
The invention relates to a phase mask for exposing a photosensitive layer in a photolithographic process. The phase mask contains a T-patterned structure composed of transparent surface segments with mutually displaced phases and a surface boundary segment whose phase is situated between the phases of the adjacent surface segments.
Such phase masks are used in photolithographic processes to produce integrated circuits, in particular to produce, conductor tracks for wiring integrated circuits.
Such conductor tracks are usually incorporated in isolator layers that are seated, directly or with the interposition of a metal layer, on a substrate that contains the integrated circuits. Such substrates usually are formed of silicon layers, while the isolator layers are formed of oxide layers, preferably made from silicon oxides.
Trenches, running in a plane or in a plurality of planes, and contact vias are incorporated in the isolator layer in order to produce the conductor tracks, it being preferred to use etching processes, in particular plasma etching processes, for this purpose.
In order to incorporate the trenches and contact vias into the isolator layer, a resist mask with a via pattern corresponding to the trenches and/or the contact vias is applied to the isolator layer. A plurality of resist masks are usually also applied successively in a multistage process, in order to incorporate contact vias and/or trenches in a plurality of planes of the isolator layer.
The individual trenches and contact vias are etched with prescribed depths through the corresponding openings in the resist masks. Thereafter, the resist masks are removed from the isolator layer. Finally, metal is deposited into the trenches and/or contact vias in order to produce the conductor tracks.
The production of resist masks on the isolator layers is performed by known photolithographic processes. The first step in this case is to apply a radiation-sensitive resist layer to the isolator layer. Radiation, in particular optical radiation, is applied to the resist layer at prescribed points by the application of stencils or the like. Thereafter, either only the exposed or only the unexposed regions of the resist layer are removed in a suitable developer. In the first case, a so-called positive resist is present, in the second case a negative resist is present. The resist layer with the via pattern thus generated then forms the resist mask for the subsequent etching processes.
In the exposure process, the beams, in particular light beams, are intended to be projected onto the surface of the resist layer as accurately as possible in accordance with a prescribed via pattern. The aim in this case is as high a resolution as possible, whose goal is to obtain as abrupt a transition as possible of exposed and unexposed points in the photoresist layer.
The exposure is performed in this case in such a way that the radiation source emits radiation that is focused via a lens onto an image plane in which the resist layer is located. Individual substrates with the resist layers applied thereon are positioned in the image plane by a stepper in the beam path of the beams emitted by the radiation source.
During the exposure, the radiation is guided through a mask, it being possible to prescribe a specific exposure pattern by the structure of the mask. The mask is usually constructed as a binary mask, for example in the form of a chrome mask. Such chrome masks have an alternating structure of transparent regions that are preferably formed by a glass layer, and nontransparent layers that are formed by the chrome layers.
A phase mask is used instead of a chrome mask in order to increase the contrast of exposed and non-exposed regions on the resist layer.
Such a phase mask can be constructed, in particular, as a half-tone phase mask. In such half-tone phase masks, the opaque layers are replaced by a semitransparent layer with a transmission of typically 6%, whose layer thicknesses are constructed such that the traversing radiation experiences a phase-angle deviation of 180xc2x0.
Furthermore, the phase mask can also be constructed as an alternating phase mask. Such an alternating phase mask has neighboring transparent regions, separated in each case by a chrome layer, which have phases respectively displaced by 180xc2x0. That is to say, the radiation traversing a transparent region is offset in phase by 180xc2x0 by comparison with the radiation that is guided through a neighboring transparent region.
An exact and contrasty optical imaging is obtained with the aid of such alternating phase masks when, in particular, the chrome layers are disposed as chrome webs running parallel to one another at a spacing. The transparent regions then likewise form webs that run between the chrome webs and have alternating phases of 0xc2x0 and 180xc2x0.
However, a structure of phase masks that have branched opaque segments constructed as chrome webs is problematical, two chrome webs forming a T-shaped structure in each case, in particular. With such a T-shaped structure, a second chrome web opens out at the longitudinal side of a first chrome web such that the first chrome web is subdivided into two partial segments. The transparent regions that surround the chrome webs are then to be constructed, in particular, as rectangular surface segments, the lengths and breadths of the surface segments being adapted in each case to the lengths of the adjacent opaque segments or parts thereof. The transparent surface segments are then preferably disposed such that in each case two surface segments situated opposite an opaque segment have phases differing by 180xc2x0. However, there are then always two surface segments remaining with phases differing by 180xc2x0 that are directly adjacent. The light beams that pass the phase mask at the boundary line of these surface segments are extinguished by interference effects, the result of this being that a non-exposed zone is obtained on the resist layer in the corresponding position.
This entails a second exposure process, with the aid of which the zone must be exposed subsequently. This constitutes an undesired additional processing step, and thus an extra expenditure on time and costs.
U.S. Pat. No. 5,840,447 discloses a phase mask that has transparent surface segments with different phases. A periodic sub-wavelength structure is provided along the boundary line between two surface segments with different phases. The sub-wavelength structure contains alternating thin layers of materials for the two adjacent surface segments. The sub-wavelength structure results in a virtually continuous transition of the refractive index during the transition from one surface segment to another. Interference is thereby prevented from extinguishing light beams that penetrate the boundary line between the surface segments.
U.S. Pat. No. 5,635,316 discloses a phase mask that has a plurality of transparent surface segments with phases of 0xc2x0 or 180xc2x0. Light beams that penetrate the boundary line between two surface segments of different phase are canceled by interference effects. A closed network of unexposed lines is obtained by a suitable configuration of the surface segments and of the boundary line structure thereby produced. A partial re-exposure of the unexposed lines is performed with the aid of a second mask in a second method step.
It is accordingly an object of the invention to provide an alternating phase mask which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has branched structures with a high contrast and a high imaging quality.
With the foregoing and other objects in view there is provided, in accordance with the invention, an alternating phase mask for exposing a photosensitive layer in a photolithographic process. The alternating phase mask contains at least two opaque segments including a first opaque segment with a longitudinal side and a second opaque segment ending at the longitudinal side of the first opaque segment. The second opaque segment subdivides the first opaque segment into two opaque partial segments on either side of an end region of the second opaque segment. Transparent surface segments are provided. One of the transparent surface segments is disposed on each side of each of the two opaque partial segments and of the second opaque segment and extend over an entire length of the opaque segments. Mutually adjacent ones of the transparent surface segments in each case have phases displaced by 180xc2x0 xc2x1xcex94 xcex1a with respect to each other, and xcex94 xcex1 being at most 25xc2x0. A surface boundary segment is disposed between two of the transparent surface segments and has a phase situated between the phases of the two of the transparent surface segments. The transparent surface segments separated by the surface boundary segment and situated opposite of the longitudinal side of the first opaque segment have phases displaced by 180xc2x0xc2x1xcex94 xcex1 with respect to each other. The surface boundary segment has a shape of an elongated rectangle with a width being substantially equivalent to a width of the second opaque segment.
The alternating phase mask according to the invention has at least two opaque segments, the first segment opening out at a longitudinal side of the second segment, and the first segment being subdivided into two partial segments on either side of an end region.
Disposed on either side of the partial segments and of the second segment over the entire length thereof in each case are two transparent surface segments that have phases displaced by 180xc2x0xc2x1xcex94 xcex1, xcex94 xcex1 being at most 25xc2x0.
The transparent surface segments that are situated opposite the longitudinal side of the first segment have a phase displaced by 180xc2x0xc2x1xcex94 xcex1 and are separated by at least one transparent surface boundary segment whose phase is between the phases of the adjacent surface segments. The phase of the surface boundary segment preferably corresponds to the arithmetic mean of the phases of the adjacent surface segments.
The surface boundary segment thus constructed prevents a negative interference of the radiation penetrating the boundary region between the adjacent surface segments. Consequently, the radiation is not canceled in the boundary region, and so the corresponding regions of the resist layer are exposed.
A re-exposure of these regions of the photoresist layer is thereby eliminated, and so a further exposure process for producing the desired structure of the resist mask can be avoided.
The surface boundary segment can be incorporated into the alternating phase mask without a large outlay on materials or costs.
Furthermore, it is advantageous that a contrasty image in a wide parameter range of the optical parameters of the imaging system is obtained with the aid of the phase mask according to the invention. In particular, a contrasty image is still obtained even when there is defocusing of the radiation penetrating the phase mask.
In accordance with an added feature of the invention, the two opaque segments complement one another and form a T-shaped structure. The T-shaped structure has a magnitude G where G=0.3xc2x7xcex/NA, xcex being a wavelength of radiation used in an exposure, and NA being a numerical aperture of an imaging system used for the exposure. The two opaque segments each have a shape of an elongated rectangle. The first opaque segment at a side opposite the end region of the second opaque segment has an indentation formed therein extending over a width of the end region. Preferably, the two opaque segments are chrome webs.
In accordance with an additional feature of the invention, xcex94 xcex1=0xc2x0.
In accordance with another feature of the invention, the surface boundary segment transverse to boundary surfaces of adjacent ones of the transparent surface segments is divided up into partial segments having different phases. Alternatively, the surface boundary segment has a homogeneous phase.
In accordance with a further feature of the invention, the transparent surface segments have a rectangular cross-section. The transparent surface elements have lengths corresponding in each case to lengths of adjacent ones of the two partial opaque segments.
In accordance with a further added feature of the invention, the phases of the transparent surface segments are 0xc2x0 or 180xc2x0, and the phase of the surface boundary segment is 90xc2x0. Alternatively, the phases of the transparent surface segments are 90xc2x0 or 270xc2x0 and the phase of the surface boundary segment is 0xc2x0.
In accordance with a concomitant feature of the invention, the surface boundary segment has a length adapted to lengths of adjacent ones of the transparent surface segments.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an alternating phase mask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.